docs: def make_pendulum(theta_max, l):
      """ Create ode solution y(t) = [theta(t), d/dt theta(t)].
  
      Initial conditions are y(0) = [theta_max, 0]. Parameter l is the
      length of the pendulum.
      """
      g_l = 9.8 / l
      def deriv(t, y):
          """ Calculate [d/d theta(t), d/dt d/dt theta(t)]. """
          theta, dtheta_dt = y
          return np.array([dtheta_dt, - g_l * gv.sin(theta)])
      y0 = np.array([theta_max, 0.0])
      return gv.ode.Integrator(deriv=deriv).solution(0.0, y0)